Phospholipid micellar particles in the form of unilamellar or multilamellar vesicles, also known as liposomes, have been used in a number of contexts as vehicles for the solubilization and delivery of active ingredient materials. Liposomes have proven in some cases to be highly advantageous in in vivo delivery systems in terms of biological compatibility, ability to isolate and solubilize otherwise insoluble and/or toxic active ingredients and ability selectively to deliver active ingredients to specific tissues or systems of interest.
Efforts have been made to solubilize ferromagnetic materials in liquid carriers in order to achieve ferromagnetic fluids since at least the early 1960's. One such example is that of magnetite, a ferromagnetic material of formula Fe.sub.3 O.sub.4 often formed by precipitation from alkaline solution of iron (II) and iron (III) chlorides. Examples of such precipitation methods include those described in Mann et al J.C.S. Chem. Comm. 1979, pp. 1067-1068, Khalafalla et al., IEEE Trans. Magnetics, Vol. MAG-16, No. 2, pp. 178-183 (March 1980) and Molday et al., J. Immunological Methods, Vol. 52, pp. 353-367 (1982). The ability of magnetite to act as a T.sub.2 relaxation enhancer in nuclear magnetic resonance has been recognized in the literature. See Ohgushi et al , J Magn. Res., Vol 29, pp. 599-601 (1978).
A number of successful techniques for solubilizing magnetite have been developed, but none prior to the present invention has been demonstrated as being suitable for in vivo use as a delivery vehicle for magnetite having extended circulation time, serum stability and biocompatibility. For example, particulate magnetite, whether uncoated or with coatings known in the prior art, is typically removed from the blood within a very short time, usually in less than one hour and in many cases within five minutes. Moreover, lack of proper solubilization of such particles may lead to aggregation in the body and resultant deleterious effects.
Solubilization of magnetite in non-aqueous solution has been achieved by ball-milling the material in the presence of a surfactant such as oleic acid, by peptization into the desired solvent with a surfactant, and by related methods. In this regard, see Charles et al., IEEE Trans. Magnetics, Vol. MAG-16, No. 2, pp. 172-177 (March 1980), Khalafalla et al., U.S. Pat. No. 3,764,540 (1973), and Reimers et al., U.S. Pat. No. 3,843,540 (1974). Characteristic of such non-aqueous, non-polar solvent suspensions of magnetite are vehicles comprising a monolayer coating of surfactant with the polar head thereof associated with the ferrite surface and the lipophilic hydrocarbon tail thereof exposed outwardly to achieve compatibility with the non-polar carrier solvent. Such compositions are not suitable for solubilization in the aqueous environment of the body.
Aqueous or polar solvent suspensions of magnetite have also been achieved. Monolayer surfactant coats of dodecylamine or dodecanoic acid on magnetite have been shown to yield dispersions of the ferromagnetic material, the latter surfactant giving a dilution-stable dispersion. Khalafalla et al., IEEE Trans. Magnetics, Vol. MAG-16, No. 2, pp. 178-183 (March 1980). Aqueous ferrofluids using petroleum sulfonates as dispersing agents have been decribed. Kelley, U.S. Pat. No. 4,019,994 (1977). The structure of such monolayer surfactant-coated particles is similar to that of the non-aqueous solubilized magnetite particles discussed above, with prevention of aggregation but retention of water solubility being achieved by virtue of shorter (less hydrophobic) hydrocarbon tails exposed to the solvent phase.
Stable aqueous suspensions of magnetite particles have also been achieved using ionic and non-ionic surfactants to produce a surface double layer. Such structures involve an inner layer of amphiphilic molecules coated on the magnetite particle as in the monolayer case, and an outer surfactant layer oriented with lipophilic tails disposed inwardly and hydrophilic heads exposed outwardly to the aqueous/polar solvent. The inner layer frequently is composed of oleic acid. Materials used as outer surfactants include fatty acids and their salts, long chain ethers or esters, and alkylaromatics such as alkylaryl polyethers. Examples of such bilayer compositions are given in Shimoiizaka, Japanese Patent No. 51-44580 (1976) and Sambucetti, IEEE Trans. Magnetics, Vol. MAG-16, No. 2, pp. 364-367 (March 1980). The outer layer surfactants which have thus far been shown to be useful in solubilizing magnetite particles are not, however, suitable for in vivo use inasmuch as they are themselves toxic and are, moreover, rapidly broken down in the blood environment potentially to allow harmful aggregation of the encapsulated materials.
Alternate means of preparing magnetite for in vivo administration include attachment of the particles to micrometer-sized carbohydrate matrices (Olsson et al., Proc. Soc. Magn. Res. Med., p. 889 (4th Ann. Mtg. Aug. 1985) and Olsson et al., Magn. Res. Imaging, Vol. 4, No. 2, pp. 142-143 (1986)) and coating of magnetite with the mucopolysaccharide chitosan (Yen et al., U.S. Pat. No. 4,285,819 (1981)). It is believed that such compositions, although possibly stable in serum, would quickly be removed from circulation by the reticuloendothelial system. Magnetically localizable polymerized liposomes containing pharmaceuticals and a ferrite material have been described in Chang, U.S. patent application Ser. No. 714,411 (March 12, 1985) now U.S. Pat. No. 4,652,257. In addition, the encapsulation of magnetite within the enclosed volume of a single bilayer phosphatidylcholine vesicle and a proposal for use in nuclear magnetic resonance spectroscopy is disclosed in Mann et al., J.C.S. Chem. Comm. 1979, pp. 1067-1068 (1979). The utility and safety of such a vesicle in this regard is not demonstrated. Furthermore, the composition described would have limited in vivo stability, making it undesirable as an imaging agent. In contrast, the delivery vehicles of the present invention have high stability in serum at 37.degree. C., are capable of extended circulation time, and are biocompatible.
The problems inherent in achieving a solubilized form of magnetite suitable for in vivo use are often applicable to other active ingredients. In particular, such ingredients may be particulate, aqueous-insoluble or toxic in nature, or it may be useful or necessary to deliver them to specific bodily sites. Furthermore, prior art delivery vehicles frequently do not have sufficient serum-stability to achieve optimal results in a safe manner.
Accordingly, the present invention addresses the need to develop improved compositions and methods capable of safely and specifically delivering active ingredients, such as therapeutic agents or diagnostic agents, including magnetic or other imaging agents, to the body in amounts effective to achieve beneficial results.